Electrolysis



Patented June 2, 1936 UNITED STATES ELECTROLYSIS Homer M. Doran and Harold D. Houghton, Great Falls, Mont assignors to Anaconda Copper Mining Company, New York. N. Y., a corpora.-

tion of Montana No Drawing. Application November 15, 1934,

Serial No. 753,124. 8 Claims. (01. 204-4) This invention relates to electrolysis, and has for an object the provision of an improved method of recovering metals from solutions of salts of the metals by an electrolytic operation involving the use of a so-called insoluble anode. The invention further contemplates the provision of an im-= proved insoluble anode adapted for use in the electrodeposition of metals, particularly in the electrodeposition of zinc. a

The invention is particularly well-suited for use in the recovery of metallic zinc of a high degree of purity by electrolysis of solution of zinc salts, and it will be described below with reference thereto. It is to be understood, however, that such reference is intended as illustrative only, and that the invention is applicable to other electrolytic operations involving the use of an insoluble anode.

In the recovery of zinc by electrolysis of a solution of a zinc salt, such as zinc sulphate, a lead anode and analuminum cathode are generally employed. Heretofore, it has been customary to employ chemically pure lead anodes, for many substances, if present in the lead from which the anode is made, will seriously afiect the purity of the zinc deposited upon the cathode. Contaminated lead anodes may also lead to a lower cur-- rent efiiciency and to retardation of the rate at which-zinc is deposited.

7 When chemically pure lead anodes are employed, the zinc deposit is generally contaminated with a small quantity of lead. The chief source of this impurity is the lead anode, from which it is dissolved or otherwise dissociated by anodic processes, and it may be included in the zinc deposit by electrodeposition from solution, by the occlusion of lead compounds in the zinc deposit, or by the adherence of lead-bearing substances to the surface of the zinc. This contamination of the zinc is reduced to a minimum if the anodes are freshly cast. After the anodes have been in use for a substantial length pf time, the lead content of the zinc deposit noticeably increases.

For some purposes, such, for example, as for use in die-casting alloys, zinc of an extremely high degree f purity, '99.99+% Zn, is required.-

drops below the standard of 99l99+% Zn. Consequently, to obtain a product containing 99.99+ zinc, it has heretofore been necessary to employ anodes of chemically pure lead, and to recast them at frequent intervals.

From a. theoretical standpoint, the amount of lead dissociated from the anode and thus made available for inclusion in the zinc deposit can be reduced by alloying with the lead used in the anode any element or compound which tends to increase its resistance to dissolution or dissociation from the anode under the influence of the anodic processes, or which tends to cause the lead compounds formed during electrolysis to adhere more closely to the anode surface. As a practi- 1.3 cal matter, however, many of the elements or compounds capable of accomplishing this effect, I such as silver, arsenic and the like, are unsatisfactory because they are themselves deposited with the zinc, or because traces of them in the solution greatly retard the rate at which zinc is deposited and/or greatly decrease the current emciency. I We have found that the amount of lead deposited with, occluded in, or adhering to the zinc produced during the electrolysis of a zinc sulphate solution may be greatly decreased by employing an anode consisting essentially of lead and cadmium and preferably containing not more than 0.5 percent cadmium. In general, we prefer 30 to employ cadmium-lead anodes containing between about 0.1 percent and 0.5 percent by weight. Anodes composed of an alloy containing between 0.2 per cent and 0.4 percent cadmium have given particularly satisfactory results. 35 Tests conducted in the laboratory and on a commercial scale have demonstrated that no substantial benefit is derived by employing in the alloy from which the anode is made more than about 0.5 percent by weight of cadmium. If the cadmium content of the anode is increased to 2.0

percent, the amount of lead included as an impurity in the zinc deposit may actually be greater than when chemically pure lead anodes are used. The invention may be employed as follows for the recovery of a metal, such as zinc, by electrolysis of a solution of a salt of the metal, such as zinc sulphate: The solution is introduced between a suitable cathode and a lead anode containing cadmium, preferably in an amount not greater 00 than 0.5 percent, and an electric current is passed therethrough. The current density, temperature, concentration of the solution, and similar conditions will depend upon the particular operation being carried out, and, in general, will be the same 55' as it any conventional anode were employed. Metal from the solution is deposited upon the cathode, and the cathode may be removed from the solution when a deposit 0! sumcient thickness has beenformed.

Anodes oi'the class described may be easily made by melting together appropriate quantities of lead and cadmium, and casting the resulting alloy in the form of an anode. Cadmium melts at about 321 C. and lead at about 327 C., and the two metals, when molten, are readily miscible in the proportions contemplated by the invention.

The addition of the cadmium to the lead eflects hardening of the lead and permits the production of anodes which'are more resistant to the corrosive influence oi the anodic processes which oc-= cur during electrolysis. Anodes of this nature may be employed for a very much longer period of time without materially contaminating the deposited metal with lead than anodes of chemically pure lead. Accordingly, anodes of the nature described herein have a long life, and their use materially reduces the expense and trouble heretofore entailed in recasting anodes of chemically pure lead. By reducing the amount of lead which dissolves in the solution, other conditions remaining unchanged, the use 01' lead-cadmium anodes eifectsincrea'sed purity of the zinc deposited and leads to increased output of high purity zinc.

We claim: g

1. In a process for recovering a metal by electrolysis of a solution of a salt of the metal and involving the use of an anode substantially insoluble in the solution, the improvement which comprises employing an anodeconsisting essentially of lead and cadmium and containing cadmium in an amount not greater than about 0.5 percent by weight.

2. In a rocess for recovering a metal by electrolysis of a solution of a salt of the metal and involving the use of an anode substantially insoluble in the solution, the improvement which comprises employing an anode consisting essentially or lead and cadmium and containing cadmium in an amount equal to about 0.1 to 0.5 percent by weight.

3. In a process for recovering a metal by electrolysis of a solution of a salt or the metal and involving the use of an anode substantially insoluble in the solution, the improvement which comprises employing an anode consisting essentially of lead and cadmium and containing cadmium in an amount equal to about 0.2 to 0.4 percent by weight. v

4. In a process for recovering zinc by electrolysis'of a solution of a zinc salt and involvin the use of an anode substantially insoluble in the solution, the improvement which comprises employing an anode consisting 01' lead and cadmium and containing not more than about 0.5 percent cadmium by weight.

5. In a process for recovering zinc by electrolysis oi. a solution of zinc sulphate and involving the use of an anode substantially insoluble in the solution oi zinc sulphate, the improvement cadmium by weight.

' HOMER M. DORAN.

HAROLD n. HOUGH'ION. 

